1. Field of the Invention
This specification relates to a technology of diagnosing an electrolytic capacitor, and particularly, to an apparatus and method for diagnosing a DC link capacitor of an inverter.
2. Background of the Invention
In general, an inverter is connected to a three-phase AC commercial power supply to smooth an AC input signal to generate a DC signal. The inverter varies a frequency and a magnitude of a voltage of the DC signal to generate a desired output voltage, thereby supplying the generated output to a motor. The inverter is widely being used in the industrial world in the aspects of energy efficiency and simplicity of output control.
An electrolytic capacitor for smoothing DC power (hereinafter, referred to as ‘DC link capacitor’) may be installed in the inverter to smooth the DC power into a predetermined level and accumulate or discharge electrical energy. Deterioration of such DC link capacitor is the most frequent cause of a breakdown of the DC link capacitor. Therefore, it is very important to diagnose a breakdown of the DC link capacitor by determining deterioration of the DC link capacitor.
FIG. 1 is a configuration view of an apparatus for diagnosing a DC link capacitor according to the related art.
An apparatus for diagnosing a DC link capacitor 112 of an inverter 110 according to the related art may be configured such that a resister 113 and a DC link capacitor 112 are connected in parallel to each other. Also, a contactor 114 may be connected in series to the resister 113. The contactor 114 may be normally open.
A voltage detector 116 may be connected in parallel to the DC link capacitor 112. A discharge time measuring circuit 117, a deterioration determining circuit 118 and an interface 119 may be connected to the voltage detector 116.
During a general operation of the inverter 110, an AC voltage (or AC power source) may be input using a breaker 100 connected to a rectifier 111 and a DC voltage (or DC power source) whose frequency is controlled by an inverter module 115 may be output to a motor 120.
Under this state, when power is cut off as the breaker 100 is open, the operation of the inverter module 115 may be stopped. Accordingly, the DC link capacitor 112 is in a charge-stored state. Here, when the contactor 114 is closed, the stored charge is discharged through the resistor 113.
If it is assumed that a capacitance of the capacitor 112 is C and a resistance of the resister 113 is R, a voltage between both poles may reduced due to the discharging according to an RC time constant. The change in the voltage may be detected by the voltage detector 116, and transferred to the discharge time measuring circuit 117. The discharge time measuring circuit 117 may measure a discharge time from the voltage change and transfer the measured discharge time to the deterioration determining circuit 118.
The deterioration determining circuit 118 may compare the discharge time with a preset reference time. When the discharge time is less than the reference time, the deterioration determining circuit 118 may output a deterioration determination signal to the outside using the interface 119.
As such, the related art diagnosing apparatus measures the change in capacitance by measuring the discharge time using a DC link voltage, and diagnoses deterioration of the capacitor accordingly.
However, the related art requires components only for diagnosis of capacitor deterioration, such as the resistance 113 and the contactor 114, which are not needed at the general operation. This may cause additional costs.